Anaerobic digestion facility nears completion in California

San Luis Obispo County, located along the Pacific Ocean in Central California midway between San Francisco and Los Angeles, is known for its breathtaking seascapes, miles of oak-studded rolling chaparral hills and prolific wine production. It is here where California’s newest anaerobic digestion facility will soon reside to convert organic waste into carbon-neutral biogas and high-grade natural compost.

Anaerobic digestion: A preferred process for disposal of organic waste in California

Utilizing anaerobic digestion as an alternative to landfills for the disposal of collected source-separated organic (SSO) waste is not new to California, nor is the conversion of organic waste into biogas and compost new to the state. California has 17 organic waste anaerobic digestion plants in operation, and another 10 in the approval/building process, including the facility at San Luis Obispo, which is scheduled to be completed and begin full production in mid-2018.

Methane emissions resulting from the decomposition of organic waste in landfills are a significant source of greenhouse gas emissions. Organic materials are also a significant portion of California's overall waste stream. Food waste alone represents approximately 17 percent of total landfill disposal within the state.

California recognizes that expanding anaerobic digestion facilities throughout the state will help reduce methane emissions from its landfills and generate biogas that can be used to create electricity or renewable transportation fuels. Consequently, the California State Legislature and Gov. Jerry Brown have enacted a number of stringent laws directed toward the collection and handling of organic municipal solid waste (MSW). California Assembly Bill No. 1826, enacted in September 2014, required businesses that generate a specified amount of organic waste per week to arrange for recycling services for that waste by April 2016 and required each jurisdiction to implement an organic waste recycling program to divert the waste collected away from landfills.

Additionally, California Assembly Bill No. 1383, enacted in September 2016, establishes targets to achieve a 50 percent reduction in the level of the statewide disposal of organic waste from the 2014 level by 2020, and a 75 percent reduction by 2025. Both AB 1326 and 1383 build upon California's leading commitments to reduce greenhouse gas emissions and air pollution statewide.

San Luis Obispo County’s unique facility for SSO waste

The San Luis Obispo County anaerobic digestion plant has been conceived in support of these initiatives. However, it is significantly more unique and streamlined in its scope of operation when compared to some of California’s other 26 anaerobic digestion facilities.

The plant will process the organic solid waste at a thermophilic temperature of 131° F, considerably higher than the lower mesophilic temperature range of most of the state’s other anaerobic digestion facilities. The higher processing temperature ensures that spores and bacteria are eliminated, completely sanitizing the organic matter during processing. And equally important, the higher temperature permits the biogas potential to be fully exploited by the time the material comes out of the digester, in effect, extracting a higher volume of carbon-neutral biogas.

It is also noteworthy that the San Luis Obispo facility is the only dry anaerobic digestion plant in California that is designed, financed, built, owned and operated by an independent engineering company, separate from a government agency or waste collection contractor. That company is Hitachi Zosen Inova AG (HZI), Zurich, Switzerland, an engineering, procurement and construction (EPC) contractor that delivers turnkey plants and system solutions for recycling organic waste.

“We started this process 10 years ago, even before [California’s] mandates were in place,” Bill Worrell, general manager, San Luis Obispo County Integrated Waste Management Authority (IWMA), says. “We recognized that an enclosed anaerobic digestion facility is really the state-of-the-art solution to handling organic MSW. It keeps everything indoors, there is no runoff and it recovers energy. It is just the best way to compost.

“As early as 2007, I was visiting anaerobic digestion plants in Germany and Switzerland, then subsequently in Canada and the United States—about a dozen plants in total,” Worrell adds. “We were looking for a long-term solution for our organics waste. So, we looked at all the options and what was available. In 2014, Hitachi Zosen Inova, a Swiss company, began offering its Kompogas anaerobic digestion technology in North America. I had visited three of the company’s plants in Europe earlier, so was familiar with their systems, and thought their technology would be a good fit for our needs at IWMA. So, this started discussions between HZI and IWMA.”

The San Luis Obispo County IWMA is a joint powers agency that covers San Luis Obispo County. Servicing the waste collection needs for five cities and much of the county unincorporated area is Waste Connections, an integrated waste services company headquartered in The Woodlands, Texas, that provides solid waste collection, transfer, disposal and recycling services. Each of the five cities and the county has its own contract with Waste Connections. Both IWMA and Waste Connections are jointly responsible to comply with California Assembly Bills 1326 and 1383 relating to organic MSW collection and disposal within the county.

“To implement the project, each of the five cities and the county agreed to 20-year waste collection contracts with Waste Connections, who would deliver the organics to the Kompogas facility, which was to be located at the Waste Connections yard near the city of San Luis Obispo,” Worrell says. “The Kompogas plant would be designed, financed, built, owned and operated by HZI, with a 20-year commitment from Waste Connections to provide the plant with the organic waste stream.”

With the agreement of all parties now in place, detailed planning, design and permitting of the new Kompogas plant could move forward.

“We worked closely with IWMA and Waste Connections on our plans, much of which entailed ensuring all participant parties, including the public, were fully informed on the processes and procedures involved with the plant operation,” William Skinner, West Coast sales manager for HZI, says. “Much of the process involved securing state permitting requirements. Part of this planning had to do with diverting green waste away from two open-air compost facilities being used by Waste Connections and moving that organic waste stream into the new Kompogas facility, once in operation. This was part of the overall strategy for supplying waste to the new facility.”

Kompogas continuous dry anaerobic digestion

The Kompogas continuous dry anaerobic digestion process is based on the dry digestion of solid bio-wastes in an oxygen-free environment. Anaerobic digestion is a multistep biological process with four fundamental steps that include hydrolysis, acidogenesis, acetogenesis and methanogenesis. Throughout this entire process, large organic polymers that make up biomass are broken down into smaller molecules by microorganisms. Upon completion of the anaerobic digestion process, the biomass is converted into biogas, as well as into digestate.

Digestate is the remaining solid and/or liquid residual from the anaerobic digestion process. The anaerobic digestion process recovers and recycles the nutrients contained in this organic material. From the produced digestate, top-grade solid compost is produced, which is made available to nurseries, farmers and market gardens. The process also produces liquid digestate, which is distributed for agricultural use as a certified organic liquid soil conditioner.

“The Kompogas process uses a horizontal plug-flow digester,” Skinner says. “The organic material is transported inside the digester, with the material moving horizontally through the system by feeding on the inlet side and discharging on the outlet side. A slowly-turning agitator ensures that the substrate is optimally mixed within the digester, and the biogas bubbles are permitted to vent for high-yield formation of methane. This facilitates the biological strength of the Kompogas anaerobic environment, enabling it to make maximum use of the organic waste’s energy potential.”  

The Kompogas digester creates an efficient microorganism environment, which separates and optimizes the different steps of biomass degradation throughout the process. This makes for stable microbiology inside the reactor.

The process recirculates approximately one-third of the digestate, rich in thermophilic microorganisms, from the output and back up front into the digester to activate and accelerate the anaerobic digestion process of the fresh material fed into the digester. This allows a perfect adjustment of the hydrolysis/acidogensis rate in the digester feeding section and facilitates high bioprocessing efficiency within the system.

If required, additional process water creates the optimal consistency for decomposition, with humidity residing around 70 percent. A specially developed heating system regulates the temperature during processing at 131° F during a retention period of approximately 14 days. The thermophilic microorganisms decompose the organic matter and produce carbon-neutral biogas. Because the digestate is completely sterilized, the system does not require any upstream pasteurization.

Power generation

One of the major points driving the final decision to move forward with the Kompogas plant was the amount of electricity that is capable of being produced on a continual basis. At full capacity, the plant is designed to produce enough biogas to generate 6.2 million kW/h annually.

“Biogas can be used either to fuel the waste collection trucks, or create electricity,” Skinner says. “What it came down to, is while Waste Connections had some CNG trucks, they did not have enough to take all the biogas that would be produced. So, we went with electricity. That way, the electricity can be used internally to power the plant, with the excess put into the grid.”

The pretreated biogas is led to a combined heat and power unit. The CHP unit is a completely containerized module featuring a gas controller, gas engine, generator, exhaust funnel, heat recovery, cooling unit, catalyst and control unit. The CHP is designed to ensure maximum possible electrical efficiency and high availability.

Exhaust air collection and treatment

One of the most important considerations during the facility planning process was controlling emissions.

“A critical factor throughout the entire development of this project was containing the release of contaminants and odors into the surrounding environment,” Skinner says. “HZI implemented very stringent processes throughout the facility to ensure this measure was intact.

“The plant also needed to comply with the California Environmental Quality Act (CEQA),” Skinner continues. “This is a statute that requires state and local agencies to identify the significant environmental impacts of their actions and to avoid or mitigate those impacts.”

The digester itself is a completely closed system, as the process operates under anaerobic conditions. Therefore, no emissions are released into the surrounding environment by the digestion plant. Exhaust air collected from the various halls is moistened with water by means of a special nozzle system operated with compressed air. Reaching humidity levels of 95 percent guarantees an optimal operation of the subsequent biofilter.

To lower the total air volume to be treated by the biofilter, the total exhaust air collected in the waste treatment hall is directed to the composting hall as inlet air. Hence, the air from the treatment hall is reused for aeration of the composting hall before it is led to the biofilter for treatment.

The biofilter consists of a large open structure with a permeable floor to allow for airflow and is filled with pieces of tree roots. After being shredded and sieved, the wood chunks offer a large surface as a breeding ground for natural microorganisms, which absorb the volatile organic compounds contained in the exhaust air. The loosely stacked biofilter results in a minimal pressure drop of the exhaust air stream.

To prevent the air from penetrating into the environment, both the treatment hall and the composting hall are kept at negative pressure.

Waste as an energy resource

For the residents of San Luis Obispo County, the implementation of this new anaerobic digestion plant represents a significant step forward towards the reduction of greenhouse gas emissions through the diversion of organic waste away from open landfills. But it is also a movement away from wasted resources towards the utilization of waste as an energy solution. The new plant makes a bold statement and champions the importance of environmental consciousness—not just to the residents of the county or state, but to the entire nation. 

The San Luis Obispo County Anaerobic Digestion Facility is being funded in part by the California Department of Resources Recycling and Recovery’s (CalRecycle) Organics Grant Program, a subset of California Climate Investments. CalRecycle funds will pay $4,000,000 for the anaerobic digestion facility in San Luis Obispo.  California Climate Investments is a statewide program that puts billions of cap-and-trade dollars to work reducing greenhouse gas emissions, strengthening the economy, and improving public health and the environment— particularly in disadvantaged and low-income communities. The Cap-and-Trade program also creates a financial incentive for industries to invest in clean technologies and develop innovative ways to reduce pollution. For more information, visit California Climate Investments.